The present invention relates generally to a surgical instrument having a locking mechanism arranged to restrict rotational movement between two components. Particular embodiment of the present invention relate to femoral sizing guides for accurately positioning a cutting block on a resected distal portion of a femur in order to locate cutting planes for preparing the end of the femur to receive a prosthetic implant. The present invention also relates to methods of using the surgical instruments, and in particular femoral sizing guides.
During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one of the patient's bones.
During performance of a joint replacement procedure, it is generally important to provide the orthopaedic surgeon with a certain degree of flexibility in the selection of the correct size of prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. In order to implant a prosthetic joint, it is commonly necessary to prepare the bone to receive the prosthesis. For a prosthetic knee joint, both the distal femur and the proximal tibia may need to be accurately resected to shape the ends of the bones to receive the implants. For preparing the distal femur as a first preparatory step, a transverse surface is formed at the distal end of the femur by performing a first resection. This resection may be located using separate instruments, not described in detail here.
Femoral knee prostheses are typically provided in a range of standard sizes. Once selected, the femoral knee prosthesis must be located and oriented to provide appropriate rotational alignment. Correct selection of the size of prosthesis and correct positioning of the prosthesis relative to the natural bone is essential to ensure natural movement of the assembled joint. In particular, the implant must be positioned to provide an appropriate gap between the femur and the tibia when the knee is in extension and in flexion, and to ensure that the surrounding tissues are correctly balanced. It is known to set the rotation of the femoral prosthesis relative to Whiteside's line, which extends from the intercondylar notch to the patella groove. Alternatively, the rotation of the femoral prosthesis may be set relative to the transepicondylar axis, which connects the high points of the epicondyles. Further anatomical reference marks may also be used.